AJS & Matchless Owners Club

Hi all,
My short stroke build is underway now, with a new bottom end in place and wheels are aligned to 1 thou. I took to Engineer to balance new piston etc, and he took one look at the rod and big end and said "you have no side play in the con rod, you'll blow the engine".

I have to go and see him again today to check this, but I suspect it may be ok, although, if the new big end was a bit bigger or smaller it may affect the side play. What sort of play should there be between the fly wheels? The only thing i can find via mr Google is 9-13thou ? seems like a lot, is this right?

For completeness, I'm talking about side to side movement of the rod on the big end, between the wheels ... NOT side to side pendulum rock at the small end.

Cheers.

Not sure if the question can be answered, not knowing the year model of your short stroke. Your profile states you have 49 G80 and 55 G80CS

Rob Harknett wrote:Not sure if the question can be answered, not knowing the year model of your short stroke. Your profile states you have 49 G80 and 55 G80CS

Oops... sorry. '56 G80CS.
m

Marty,

There must be some side play between rod big end and flywheel. If there is none or too little, the oil cannot escape and the oil and bearing will overheat which may damage and possibly seize the bearing.

The 9 to 13 thou you refer to are for the single cylinder models around 1949, also for the (early) AJS 7R.

Oil flow was increased on later engines, so you would not want to have smaller clearances on your engine. Phil Irvings “Tuning for Speed”: “The correct amount of side clearance varies according to the design, but is rarely of great importance, 0.010 – 0.020 inch being about right in most cases.”

How much (or little) is it on your engine?

Bjørn

Thanks Bjorn,

I'm not sure how much play mine has as yet. The engineer has it, but will ask that he checks the clearance prior to pulling it apart. I'm loath to undo my big end, as I got it trued within 1 thou!
Cheers.
Marty